Conventional network systems and server systems include a number of printed wiring boards (printed boards) connected to one another for the purposes of high integration and high-speed transmission.
A method for connecting a number of printed wiring boards generally uses connectors as depicted in FIGS. 9A-9C, for example.
Specifically, as illustrated in FIG. 9A, in conventional electronic device 100, printed wiring board (main board) 101 on which parts are mounted and printed wiring board (sub board) 102 include stack connectors 103 and 104, respectively.
As illustrated in FIG. 9B, coupling of stack connectors 103 and 104 to each other communicably connects printed wiring boards 101 and 102, so that electronic device 100 shown in FIG. 9C is formed.
However, connection of printed wiring boards via connectors, such as stack connectors 103 and 104, has the following problems (1) to (4):
(1) In connector mounting, the number of pins of each connector restricts the wire connection between main board 101 and sub board 102, so that freedom in design is lowered;
(2) Connection via the connectors increases the transmission loss of signals between printed wiring boards 101 and 102;
(3) The height of electronic device 100 increases by the heights of connectors 103 and 104 to restrict the part-mounting structure of electronic device 100, making it difficult to enhancing the freedom in design and to save the space; and
(4) Because space is required to install connectors 103 and 104, it is difficult to make electronic device 100 and printed wiring boards 101 and 102 higher in integration and smaller in size.
As the solution to the above problems (1) to (4), there has been provided a multi-layer rigid/flex printed board (hereinafter sometimes called multi-layer printed wiring board) formed of a number of multi-layer rigid board that are connected to one another by flexible boards on each of which circuits are formed on a flexible board made of a polyimide resin film that is a material high in flexibility (see Patent Reference 1 and 2 below, for example).
[Patent Reference 1] Japanese Patent Application Laid-Open (KOKAI) No. 2005-353827
[Patent Reference 2] Japanese Patent Application Laid-Open (KOKAI) No. HEI 6-320671
However, such a conventional multi-layer printed wiring board including a polyimide resin film reduces the flexibility in accordance with increase in the number of layers laminated for the purpose of increasing the number of signal lines.
Further, if a signal line is formed on the outermost layer or a signal line on the outermost layer is coated as the techniques disclosed in the above Patent Reference 1 and 2, impedance control that maintains the impedance of a signal through these signal lines to be constant (e.g., 50 Ω) cannot be accomplished so that signals cannot be transmitted at a high speed.